In general, I think a nicely built outbuilding that’s in keeping with the rest of the property will always add some value. Whether you’ll break even or make a few quid is another thing.
My workshop is a small ~4.5 x 4m brick-built, with the bricks matched to the house, as was the UPVC window etc. Only thing that doesn’t match is the white powder-coated steel door, which is something I’ve been meaning to address for the last two summers. Maybe this summer...
I’ve no idea how much it will add to value of the house, but it was never really a consideration. I’m not planning on moving for years, and having the luxury of a comfortable and fairly secure workshop was worth every penny. Had it for about three years now, and my only regret is that I didn’t do it 15 years ago.
My advice would be that if you’re going to enjoy it, just do it.
Cheers
Stu
PS. I now know a lot more about space management than I ever did before.